Dynamically re-classifying data in a shared cache

ABSTRACT

In one embodiment, the present invention includes a method for determining if a state of data is indicative of a first class of data, re-classifying the data from a second class to the first class based on the determination, and moving the data to a first portion of a shared cache associated with a first requester unit based on the re-classification. Other embodiments are described and claimed.

BACKGROUND

In multi-core processors or other processing systems, each core may havean associated cache memory, i.e., a private cache accessible only bythat core. Furthermore, a shared cache memory, accessible to all of thecores, may be provided to extend cache capacity. Cache access time canbe affected by propagation delays present in electrical circuitry. Ingeneral, cache access time may increase in proportion to such physicalproperties as the distance between the cache and an accessing logic, thewidth of a data interconnect, and so forth.

The optimal cache design for a multi-core architecture is a currentresearch issue, and one of the most basic questions is whether a largecache should be organized into a single (e.g., banked) shared cache, orinto private caches for the cores. A shared last-level cache can performpoorly when the private data of each thread's working set exceeds acore's private cache and data has to be repeatedly re-fetched from aremote piece of the shared cache. A private last-level cache can performpoorly when threads share most of their working sets. Thus an optimalchoice depends on the total amount of cache capacity, an application'sworking set size, and the application's data sharing patterns. Bothshared and private access patterns are expected to occur in futurerecognition, data mining, and synthesis (RMS) applications, as anexample.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a processor in accordance with anembodiment of the present invention.

FIG. 2 is a flow diagram of a method in accordance with one embodimentof the present invention.

FIG. 3 is a flow diagram of a method for re-classifying data inaccordance with an embodiment of the present invention.

FIG. 4 is a flow diagram of a search policy technique in accordance withone embodiment of the present invention.

FIG. 5 is a block diagram of a system in accordance with one embodimentof the present invention.

DETAILED DESCRIPTION

Embodiments relate to processor-based systems, and more particularly tosuch systems including cache memory.

Embodiments of the present invention may be used to dynamicallyclassify/re-classify data in a shared cache to effect differing locationpolicies. In this way, an adaptive last-level cache (LLC) such as may beused in connection with a chip multiprocessor can be a hybrid of aprivate and shared cache. That is, while the cache may be a shared,segmented last-level cache, it can automatically detect if data isprivate and use at least some of its storage as private cache. In thisway, data that is accessed by only one core at a time may be placed nearthat core, reducing average travel distance of memory accesses, which inturn reduces on-die interconnect traffic and thus reduces energyconsumption and improves performance.

Thus in various embodiments, a scalable cache architecture may adaptitself to application needs. Based on state information associated withgiven data (e.g., cache line state), it may be determined if a line inthe LLC would likely benefit from being treated as private (e.g., theline is being repeatedly accessed by a single core). Based on thisdetermination, the cache line may be classified as private and moved tothe bank of the LLC closest to the core that is using it. Futureaccesses to this “private” data by another core may trigger movement ofthe data to the bank closest to that new core. As will be describedfurther below, embodiments may use state information associated withcache lines to detect if “private” data would benefit from beingre-classified as shared (e.g., if it is being moved too often). Thecombination of these mechanisms allows the classification of data tochange during program execution as an application goes from one phase toanother (e.g., a serial region in which the data may be classified as“private,” to a parallel region in which the data may be classified as“shared”). In this way, at least portions of a shared cache may beautomatically reconfigured to act as an extension of a core's privatecache.

Referring now to FIG. 1, shown is a block diagram of a processor inaccordance with an embodiment of the present invention. As shown in FIG.1, processor 100 includes multiple cores, namely cores 110 ₀-110 _(N)(generically core 110). Each core 110 may have a corresponding privatelevel one (L1) cache, namely private caches 120 ₀-120 _(N) (genericallyprivate cache 120). Note that while only a single level of private cacheis shown in the embodiment of FIG. 1, the scope of the present inventionis not limited in this regard and in other embodiments multiple levelsof private caches may be present. In many embodiments, it may be assumedthat all portions of processor 100 shown in FIG. 1 are part of a singlesubstrate, although the scope of the present invention is not solimited.

Still referring to FIG. 1, an interconnect 130 couples cores 110 andtheir corresponding private caches 120 to a shared cache 150, alsoreferred to herein as a last level cache (LLC), via a cache controller160 of shared cache 150. Note that while described as a bankedembodiment, in other implementations shared cache 150 may be segmentedin another manner. For ease of discussion, the terms LLC and sharedcache may be used interchangeably, although in many embodiments multiplelevels of shared caches may be present. In the embodiment of FIG. 1,shared cache 150 may be segmented into multiple banks, namely banks 140₀-140 _(N) (generically bank 140). As shown in FIG. 1, physically eachbank 140 may be located closest to a given one of cores 110. Thusphysically there is a different distance between each bank 140 and eachcore 110. Accordingly, each core 110 has a single bank 140 that isclosest to it. As will be described further below, in variousembodiments at least certain data closely associated with a given coremay be stored in the bank 140 closest to it. Typically, data associatedwith a given address exists in a single bank of shared cache 150. Thatis, generally a single copy of the data exists in shared cache 150.Generally, an address hashing function may be used to determine where agiven piece of data is to be stored within shared cache 150.

In various embodiments, data stored in locations (i.e., cache lines) ofshared cache 150 may be classified as roaming or non-roaming.Designation of data as roaming or non-roaming may correspond to alocation protocol that is used to determine in which bank or otherportion of shared cache 150 that the corresponding cache line is to bestored. That is, multiple location protocols may be used to store data,depending on the data type. Non-roaming data may be stored according toan address hash function. In other words, this location protocolcorresponds to a protocol favoring shared data, where the hash functiondictates in which bank the data is to be stored. However, roaming datamay be stored in the bank 140 that is closest to the last one of cores110 to use the data corresponding to the address or cache line. In otherwords, this location protocol corresponds to a protocol favoring privatedata, where the proximity of the bank to the using core dictateslocation. That is, although a single copy of any roaming cache line mayexist in shared cache 150 at any given point in time, the cache line maybe stored in any one of banks 140, depending on which one of cores 110last touched the cache line. In many implementations, each cache linemay include an indicator (i.e., a roaming indicator) to indicate whethera given cache line is roaming. In one embodiment, a roaming flag (i.e.,an R bit) may be present to indicate roaming status. Data classified asroaming may be classified in this way based on a usage pattern of thedata. That is, roaming data may move amongst banks when a usage patternindicates that a given core (i.e., a core closest to a bank in which theroaming data is located) is likely to repeatedly access the roaming datato the exclusion of other cores. In contrast, data may be classified asnon-roaming when usage patterns indicate that the data is likely to benext accessed by any given one of the cores.

To further effect handling of roaming data, each bank 140 may include aroaming data pointer (RDP) structure 145 ₀-145 _(N) (generically RDP145). RDP's 145 may act as a mechanism to locate roaming data. In oneembodiment, each RDP 145 includes a plurality of storage locations orentries 146 ₀-146 _(N) (generically entry 146), each corresponding to aroaming cache line. For example, with reference to FIG. 1, entry 1460 isused to point to an entry 141 _(N) in bank 140 _(N). Similarly, entry146 ₂ is used to point to an entry 141 ₂ in bank 140 ₁. Further, entry146 _(N) is used to point an entry 141 ₁, also in bank 140 ₁. Each entry146 may include various information to indicate the status and locationof the corresponding roaming cache line. Specifically, in one embodimenteach entry 146 may include a valid indicator, an address tag, and anidentifier corresponding to the given bank 140 that includes the roamingcache line. Of course, additional and/or different information may bepresent in other embodiments. Using this information, a requester may beable to search for and locate requested roaming data. In theimplementations described herein, it may be assumed that RDPs 145 usethe same address hashing function that is used for non-roaming data inthe shared cache 150, although the scope of the present invention is notlimited in this regard. Furthermore, in other embodiments RDPs 145 maybe located in different hardware structures than banks 140.

In some embodiments, inclusion between private caches 120 and sharedcache 150 may be accommodated such that shared cache 150 is inclusive ofall data present in private caches 120. However, other embodiments maybe used in connection with a directory-inclusion scheme in which ashared cache includes information to identify the location of data in anL1 cache. Note that while described with this high-level implementationin the embodiment of FIG. 1, it is to be understood that a processor inaccordance with an embodiment of the present invention can include manyother features and may be differently configured.

To effect classification, location, movement, and searching of datawithin a cache hierarchy, various hardware and software mechanisms maybe present. In many implementations, hardware such as cache controller160 may be present in or associated with shared cache 150 to enableidentification and control of roaming data. In various embodiments,cache controller 160 may automatically determine whether data in sharedcache 150 should be classified as roaming or non-roaming, e.g., based onstate information associated with the data. In various applications,during different phases of operation, a given piece of data may beaccessed in different manners. For example, a given piece of data may beaccessed in a shared manner in certain application phases, yet accessedin a private manner in other phases. Accordingly, embodiments may beused to dynamically re-classify non-roaming data as roaming data, andvice versa.

Referring now to Table 1 below, shown is a listing of state fields orsegments present in a shared cache entry in accordance with anembodiment of the present invention.

TABLE 1 State Tag Data Sharing vector Owner Roam Flag Reuse counter CoreCounterSpecifically, the fields present include a state field, which mayinclude indicators that correspond to a given cache coherence protocol,such as a mutual, exclusive, shared, and invalid (MESI) or a mutual,shared, and invalid (MSI) protocol. Other fields include a tag field anda data field that includes the data of a given cache line. A sharingvector field may include information to identify each core that includesa copy of the line in its private cache. An owner field is used toidentify an owner (core with exclusive permission) of the line. A roamflag or indicator is used to indicate that the corresponding cache lineis in a roaming state. A reuse counter field may correspond to a countof a number of times in a row that a given core accesses the data, whenthe cache line has been classified as roaming. In various embodimentsthe reuse counter field may correspond to a saturating counter whichsaturates at a given number N of consecutive accesses by a given core.In some embodiments, additionally a core counter field may be present totrack the number of cores that have touched the cache line. Note thatthis field may be reset when any core touches the line a sufficientnumber of times in a row (i.e., if the reuse counter field hits itsmaximum value). While shown with these particular state fields in Table1, it is to be understood the scope of the present invention is notlimited in this regard.

On an L1 miss, if the requested data is found in the LLC and isnon-roaming according to the roaming indicator, the state of the cacheline, including for example, the cache coherency state and sharing stateof the line (e.g., sharing vector and owner fields) may be examined todetermine if the current requester (e.g., given L1 cache) is the same asthe requester that touched the data last. More specifically, if the lineis in a read-only state (i.e., no L1 cache currently has permission towrite the line), it may be determined whether there is only one readerand if that reader is the requesting L1 cache (e.g., according to thesharing vector field). This case can arise because in some embodiments,L1 caches do not notify the last-level cache if they evict a line in aread-only state. If none of the L1 caches are known to have the line, itmay next be determined if the requesting L1 cache was the last one tohave the line. If either of the above conditions holds, this is a strongindicator that the data is private, exhibits temporal locality, but doesnot fit in the requesting L1 cache. Thus, this data may benefit frombeing treated as private, and may be re-classified as roaming. Note thatwhen data is re-classified as roaming, it is moved from its conventionallocation in the shared cache to the bank closest to the L1 cache thatjust requested it, and the roam flag field is set.

Referring now to FIG. 2, shown is a flow diagram of a method inaccordance with one embodiment of the present invention. Specifically,method 200 may be used to determine if data being requested by a givencore should be re-classified as roaming data. In some implementations,method 200 may be performed by a cache controller of a shared cache suchas an LLC. Method 200 may begin by receiving a request for data that isin the last-level cache and is currently non-roaming (block 205). Such arequest may come from a given processor core (via its L1 cache) thatrequests a cache line from a shared cache (i.e., the requested cacheline is not present in the private cache associated with the core).While described as a core that requests data from a shared cache, thescope of the present invention is not so limited. That is, in manyembodiments other agents of a system may request data from a sharedcache. Referring still to FIG. 2, control passes to diamond 210, whereit may be determined whether the cache line requested is shared by atmost a single L1 cache. In some embodiments, this may be determined byanalysis of state information, e.g., in a sharing vector field. If theline is shared by multiple caches, control passes to block 230, whereinformation regarding the requester may be stored, e.g., in the sharingvector field of the cache line (block 230). Further, a non-roaming replymay be generated (block 250).

Referring still to FIG. 2, if it is determined that the cache line isshared by at most a single L1 cache, control passes to diamond 220.There, it may be determined whether the current requester is the same asthe previous requester (diamond 220). For example, this determinationmay be performed by a comparison between the current requester and aprevious requester's identifier in a sharing vector field of the cacheline. If the request is from a previous requester, control passes toblock 230, discussed above. If instead it is determined that the currentrequester is the same as the immediate prior requester, control passesto block 240.

At block 240 a reply may be sent that provides the data to therequester. Furthermore, the cache line may be promoted to roaming state.That is, a roaming indicator in a state field of the cache line may beset to indicate the status of the cache line as including roaming data.While not shown in the flow diagram of FIG. 2, it is to be understoodthat when a given cache line is promoted to roaming status, the cacheline may be moved from a current shared cache location to a bank orother portion of the shared cache closest to the requesting core. Inthis way, improved access times and reduced power consumption may berealized when the requesting core seeks the data again.

Note that in various implementations all data may be classified asnon-roaming by default. That is, on a shared cache miss, data is broughtinto the cache as non-roaming (i.e., the roaming flag field is cleared),and the data is brought into a given bank according to, e.g., an addresshash function. As described above, during execution of an application,different phases may access the data differently. Accordingly, invarious embodiments methods may be present to determine if data that isclassified as roaming data should instead be re-classified asnon-roaming data. Referring now to FIG. 3, shown is a flow diagram of amethod for re-classifying data in accordance with an embodiment of thepresent invention. As shown in FIG. 3, method 300 (which may beperformed by a cache controller, in some embodiments) may begin byreceiving a request from an RDP (block 305). Note that such a requestcomes from the RDP that includes an entry that points to a bankincluding the requested roaming cache line. As shown in FIG. 3, fromblock 305 control passes to diamond 310. There, it may be determinedwhether a number of accesses from a current owner core (i.e., the coreassociated with the bank including the roaming data) exceeds a threshold(diamond 310). In various embodiments, the determination made in diamond310 may be done by accessing a reuse counter field of the cache line,which acts as a saturating counter to track the number of times in a rowthat a given cache line is accessed by the same core. If the counter hasreached its threshold, a reply message may be sent in which the data'sstatus remains as roaming and it is moved from its current location to anew bank, i.e., a bank closest to the currently requesting core (block330). While not shown in FIG. 3, the RDP including a pointer for thisline may be updated accordingly.

Referring still to FIG. 3, if instead at diamond 310 it is determinedthat the number of accesses by the current owner core does not exceedthe threshold, control passes to block 320. There a reply message may besent to the requesting core with the data (block 320). Furthermore, thecache line may be re-classified as non-roaming data. That is, if therequest is by another core before the reuse counter saturates (i.e.,meets the threshold), the data may benefit from re-classification asnon-roaming status. Accordingly, the roaming field flag may be reset toindicate that the line includes non-roaming data. Note that in someembodiments, when data is re-classified as non-roaming, the cache linemay be moved from its current bank to another bank as determined by anaddress hashing function, although the scope of the present invention isnot so limited. Furthermore, an RDP entry for the cache line may bedeallocated when it is re-classified as non-roaming. Accordingly, method300 may be used to detect if roaming data is no longer being accessed ina private manner.

An alternate embodiment for detecting if roaming data is no longer beingaccessed in a private manner is to use the reuse counter field andimplement an additional counter per cache line (i.e., the core counterfield). The core counter field may be used to track the number of coresthat have touched the cache line. However this counter may be reset whenany core touches the line enough times in a row (i.e., if the reusecounter hits its threshold value). If the core counter overflows, thedata may be re-classified as non-roaming. Note that this embodiment ismore conservative about re-classifying data as non-roaming, but uses anadditional counter per cache line as well as additional control logic.The selection of one of these approaches may depend on hardware cost andtarget application behavior.

When data is classified as roaming, it may be stored in a differentcache location (e.g., a different bank) than the bank that it would bestored in according to a given cache location scheme such as an addresshashing function. Accordingly, embodiments may provide policies for datasearching to maximize the benefits of reduced searching costs forroaming data in a shared cache in accordance with an embodiment of thepresent invention. Referring to FIG. 4, shown is a flow diagram of asearch policy technique in accordance with one embodiment of the presentinvention. As shown in FIG. 4, method 400 may begin on an L1 cache miss(block 405). That is, method 400 may be initiated when data requested bya given core is not present in a private cache associated with the core.In such circumstances, control passes from block 405 to diamond 410. Atdiamond 410, it may be determined whether the data is present in thelast level cache bank closest to the L1 cache (and requesting core). Ifso, control passes to block 420, where the data is returned to the L1cache. Note that this search and access corresponds to a cache hit fordata in the closest LLC bank to the requesting core. Diamond 410 andblock 420 thus act as the fastest path for shared cache hits.

Referring still to FIG. 4, if it is determined instead at diamond 410that the data is not present in the closest cache bank, it may bedetermined in which bank the data is present (block 430). In someimplementations, the bank may be determined by computing an addresshashing function, which provides an indication of the bank in which thedata would be stored. From block 430, control passes to diamond 440.There, it may be determined if the data is present in the computed bank(diamond 440). If so, a cache hit occurs and data may be returned to theL1 cache (block 450). If not, control passes from diamond 440 to diamond460. There, it may be determined whether an entry exists in the RDPassociated with the computed bank for the data (diamond 460). Note thatin some embodiments, the determinations of diamonds 440 and 460 may beperformed in parallel. If an entry is present in the RDP, theinformation in the entry may be used to forward a request to the givenbank that includes the data and when accessed, the data may be returnedto the L1 cache associated with the requesting core (block 480). Ifinstead at diamond 460 it is determined that there is no entry presentin the RDP for the requested data, a cache miss occurs (block 470).Accordingly, various mechanisms to request the missing data from lowerlevels of a memory hierarchy, such as a more remote cache, system memoryor other location may be performed. While described with this particularimplementation in the embodiment of FIG. 4, it is to be understood thescope of the present invention is not limited in this regard.

For example, an alternate search policy may be to search the bankaccording to the address hashing function first. However, this cangreatly reduce the latency and energy benefits of roaming data. Stillfurther implementations may use a hardware predictor (e.g., bloom-filteror partial tag) to determine whether to check the local bank first orthe conventional bank first.

Note that in certain applications, e.g., RMS workloads, shared data isalmost always shared by all threads. Therefore, a binary classificationscheme (i.e., roaming or non-roaming) may be used. However, in otherimplementations, a scheme that attempts to capture sharing by a subsetof threads may be used.

Note that when a line is re-classified as roaming and moved to theclosest bank, this may trigger an eviction of another cache line. If theevicted line is non-roaming, it may be evicted conventionally. If theevicted line is roaming, it may be re-classified as non-roaming. Then,the line may be attempted to be moved back to a bank according to anaddress hashing function. Note that without this scheme, each threadwould be limited in how much roaming data it could have in the sharedcache at any one time (i.e., one bank's worth). If only a small set ofthreads is actively using the shared cache, this may be wasteful. Thusembodiments allow each thread in the system to potentially use theentire shared cache, even for roaming data (although data outside theclosest bank may be classified as non-roaming). When attempting to movedata back to another bank, this might trigger an eviction of anotherline. In one embodiment, a typical replacement policy may be used tochoose the victim, and if the victim is roaming, it is evicted from thechip (i.e., it is not re-classified as non-roaming). In anotherembodiment, re-classification of a roaming line to non-roaming is notallowed to trigger an eviction of a roaming line. Instead, if the onlypotential victims are roaming lines, the move fails and the data thatwas re-classified is evicted.

Thus in various embodiments, fast searching for roaming data may berealized. Because in many applications, a significant amount of datawill be classified as roaming, many shared cache hits will be to thebank closest to the requesting core and reduce average memory latency.These hits will be fast and consume little energy since the requestsonly need to travel a short physical distance through an on-dieinterconnect. Further, these requests also consume little on-dieinterconnect bandwidth, making a system more scalable in terms of numberof cores.

Further embodiments of the present invention may adapt to anapplication's memory access behavior. That is, dynamic detection as towhether each piece of data is currently exhibiting properties of privateor shared data occurs with different policies provided for each dataclass. In other words, embodiments may automatically classify the dataas shared or private, and allow this classification to change over timewithout any programmer effort, compiler changes, or instruction setarchitecture support. Further still, there is no need to keep multiplecopies of data in the shared cache, maximizing the amount of usablecache space. Embodiments thus dynamically classify and re-classify datawithout the need for programmer feedback, duplicating data in thelast-level cache, or requiring classification of entire pages in thesame way, i.e., private or shared.

Embodiments may be implemented in many different system types. Referringnow to FIG. 5, shown is a block diagram of a system in accordance withan embodiment of the present invention. As shown in FIG. 5,multiprocessor system 500 is a point-to-point interconnect system, andincludes a first processor 570 and a second processor 580 coupled via apoint-to-point interconnect 550. As shown in FIG. 5, each of processors570 and 580 may be multicore processors, including first and secondprocessor cores (i.e., processor cores 574 a and 574 b and processorcores 584 a and 584 b). Note that each of the cores may include privateL1 caches. Furthermore, LLCs 575 and 585 may be associated with thecores. In various embodiments, LLCs 575 and 585 may implementclassification and associated policies in accordance with an embodimentof the present invention.

First processor 570 further includes point-to-point (P-P) interfaces 576and 578. Similarly, second processor 580 includes P-P interfaces 586 and588. As shown in FIG. 5, memory controller hubs (MCH's) 572 and 582couple the processors to respective memories, namely a memory 532 and amemory 534, which may be portions of main memory (e.g., dynamic randomaccess memory (DRAM)) locally attached to the respective processors.

First processor 570 and second processor 580 may be coupled to a chipset590 via P-P interconnects 552 and 554, respectively. As shown in FIG. 5,chipset 590 includes P-P interfaces 594 and 598. Furthermore, chipset590 includes an interface 592 to couple chipset 590 with a highperformance graphics engine 538. In one embodiment, an Advanced GraphicsPort (AGP) bus 539 may be used to couple graphics engine 538 to chipset590. AGP bus 539 may conform to the Accelerated Graphics Port InterfaceSpecification, Revision 2.0, published May 4, 1998, by IntelCorporation, Santa Clara, Calif. Alternately, a point-to-pointinterconnect 539 may couple these components.

In turn, chipset 590 may be coupled to a first bus 516 via an interface596. In one embodiment, first bus 516 may be a Peripheral ComponentInterconnect (PCI) bus, as defined by the PCI Local Bus Specification,Production Version, Revision 2.1, dated June 1995 or a bus such as a PCIExpress bus or another third generation input/output (I/O) interconnectbus, although the scope of the present invention is not so limited.

As shown in FIG. 5, various I/O devices 514 may be coupled to first bus516, along with a bus bridge 518 which couples first bus 516 to a secondbus 520. In one embodiment, second bus 520 may be a low pin count (LPC)bus. Various devices may be coupled to second bus 520 including, forexample, a keyboard/mouse 522, communication devices 526 and a datastorage unit 528 such as a disk drive or other mass storage device whichmay include code 530, in one embodiment. Further, an audio I/O 524 maybe coupled to second bus 520. Note that other architectures arepossible. For example, instead of the point-to-point architecture ofFIG. 5, a system may implement a multi-drop bus or another sucharchitecture.

Embodiments may be implemented in code and may be stored on a storagemedium having stored thereon instructions which can be used to program asystem to perform the instructions. The storage medium may include, butis not limited to, any type of disk including floppy disks, opticaldisks, compact disk read-only memories (CD-ROMs), compact diskrewritables (CD-RWs), and magneto-optical disks, semiconductor devicessuch as read-only memories (ROMs), random access memories (RAMs) such asdynamic random access memories (DRAMs), static random access memories(SRAMs), erasable programmable read-only memories (EPROMs), flashmemories, electrically erasable programmable read-only memories(EEPROMs), magnetic or optical cards, or any other type of mediasuitable for storing electronic instructions.

While the present invention has been described with respect to a limitednumber of embodiments, those skilled in the art will appreciate numerousmodifications and variations therefrom. It is intended that the appendedclaims cover all such modifications and variations as fall within thetrue spirit and scope of this present invention.

1. A method comprising: determining if a state of data is indicative ofa first class of data to be located in a shared cache according to afirst location protocol; re-classifying the data from a second class ofdata located in the shared cache according to a second location protocolto the first class based on the determination; and moving the data to afirst portion of the shared cache associated with a first requester unitbased on the re-classification.
 2. The method of claim 1, wherein movingthe data comprises moving the data from a current bank of the sharedcache to a first bank of the shared cache, wherein the first bank is aclosest bank to the first requester unit.
 3. The method of claim 1,further comprising updating a roaming indicator in a cache lineincluding the data to indicate that the data is of the first class. 4.The method of claim 1, further comprising updating a first counter ifthe first requester unit requests the data from the first portion of theshared cache.
 5. The method of claim 4, further comprising:re-classifying the data from the first class to the second class if adifferent requester unit requests the data before the first counterreaches a threshold level; and moving the data from the first portion ofthe shared cache to a different portion of the shared cache.
 6. Themethod of claim 1, further comprising: updating a second counter when adifferent requester unit requests the data; and re-classifying the datafrom the first class to the second class if the second counteroverflows.
 7. The method of claim 1, further comprising searching thefirst portion of the shared cache if requested data is not present in afirst private cache associated with the first requester unit.
 8. Themethod of claim 7, further comprising searching a data structure of asecond portion of the shared cache based on the second locationprotocol, if the requested data is not present in the first portion ofthe shared cache; and searching a pointer structure associated with thesecond portion based on the second location protocol, wherein the secondlocation protocol comprises a shared location protocol.
 9. The method ofclaim 1, further comprising storing a pointer to the first portion in acurrent portion of the shared cache in which the data is located, andmoving the data without retaining a copy of the data in the currentportion.
 10. The method of claim 1, further comprising dynamicallyre-classifying the data from the second class to the first class basedon a usage pattern of the data.
 11. The method of claim 10, whereindynamically re-classifying the data comprises analyzing stateinformation associated with the data including a sharing indicator toindicate whether one or more private caches coupled to the shared cacheinclude a copy of the data.
 12. The method of claim 1, furthercomprising re-classifying the data without user support.
 13. The methodof claim 2, further comprising: evicting an entry from the first bank ofthe shared cache; re-classifying data of the evicted entry from thefirst class to the second class; and moving the data of the evictedentry to a second bank of the shared cache according to the secondlocation protocol.
 14. An apparatus comprising: a shared cache memoryhaving a plurality of portions, each of the portions to store entries,wherein each entry is to include state information for data of theentry, the state information including a sharing indicator to indicatewhether one or more private caches coupled to the shared cache memoryinclude a copy of the data; and a controller coupled to the shared cachememory to analyze the state information of an entry and to automaticallydetermine if the entry includes data of a first type or data of a secondtype with respect to a location protocol for placement of entries in theshared cache memory.
 15. The apparatus of claim 14, wherein each entryis to include a classification indicator field to indicate whether thecorresponding entry includes data of the first type or data of thesecond type.
 16. The apparatus of claim 14, further comprising aplurality of pointer structures each associated with one of the portionsof the shared cache memory, wherein each pointer structure is to includeat least one pointer entry to point to the portion of the shared cachememory in which a requested data element of the first type is located.17. The apparatus of claim 14, wherein each entry in the shared cachememory is to further include a first counter field to indicate a numberof consecutive accesses to the entry by a requester core.
 18. Theapparatus of claim 17, wherein each entry in the shared cache memory isto further include a second counter field to indicate a number ofaccesses to the entry by different cores.
 19. The apparatus of claim 14,wherein the data of the first type comprises roaming data and the dataof the second type comprises non-roaming data, wherein the roaming datais to be stored in a portion of the shared cache memory according to aprivate location protocol and the non-roaming data is to be stored in aportion of the shared cache memory according to a shared locationprotocol.
 20. The apparatus of claim 14, wherein the shared cache memoryis to first search for a data element requested by a first requesterunit in a first portion of the shared cache memory, wherein the firstportion is the one of the plurality of portions closest to the firstrequester unit.
 21. The apparatus of claim 20, wherein the shared cachememory is to determine if the data element is present in a secondportion of the shared cache memory if the data element is not present inthe first portion, wherein the second portion is selected for searchingbased on a shared location protocol.
 22. The apparatus of claim 21,wherein the shared cache memory is to determine if the data element ispresent in a third portion of the shared cache memory, wherein the thirdportion is selected for searching based on a hit in a pointer structureassociated with the second portion.
 23. An article comprising amachine-readable storage medium including instructions that if executedby a machine enable the machine to perform a method comprising: firstsearching a first portion of a cache memory for data not present in afirst private cache associated with a first requesting unit; and secondsearching a second portion of the cache memory if the data is notpresent in the first portion, wherein the second portion is selected viaa shared location protocol, wherein the second searching includessearching a data structure of the second portion and a pointer structureassociated with the second portion.
 24. The article of claim 23, whereinthe method further comprises forwarding a request to a third portion ofthe cache memory based on a hit in the pointer structure.
 25. Thearticle of claim 24, wherein the method further comprises returning datato the first requesting unit from the third portion.
 26. The article ofclaim 23, further comprising requesting the data from a memory hierarchycoupled to the cache memory if a hit does not occur in the pointerstructure.
 27. A system comprising: a first cache having a plurality ofportions, each of the portions to store entries, wherein each entry isto include a classification indicator field to indicate whether thecorresponding entry includes data stored according to a first locationprotocol; and a dynamic random access memory (DRAM) coupled to the firstcache.
 28. The system of claim 27, further comprising a plurality ofprivate caches coupled to the first cache, and wherein a data request isto be first sent to a closest portion of the first cache to a privatecache that requests the data.
 29. The system of claim 27, wherein thefirst cache is to update the classification indicator field of an entryincluding data stored according to the first location protocol based ona change in usage of the data, wherein the first location protocolcomprises a private location protocol.
 30. The system of claim 29,wherein the update is to indicate that the data is stored according to asecond location protocol, wherein the second location protocol comprisesa shared location protocol.